PulseEffects, the audio effects tool for PulseAudio, was updated to version 4.0 recently, with subsequent bug-fix releases to version 4.0.4, introducing new features like an option to change the number of its PulseAudio equalizer bands, or use a custom color for the spectrum, among others.

PulseEffects allows using multiple audio input and output effects on your system (which needs to use PulseAudio), including reverberation, stereo enhancer, limiter, auto volume, as well as a 30-band system-wide equalizer.
The application was updated to version 4.0.0 about 10 days ago, and since then there were 4 bug-fix releases, but the 4.x versions weren't available on FlatHub until recently (the current version on Flathub is 4.0.2 so it's 2 bug-fix versions behind).

With version 4.0, PulseEffects uses a different preset format, which means users will have to create new presets. "I know it sucks but it was necessary", mentions the PulseEffects developer. Also, with 4.0, PulseEffects has moved from Python to C++.

The system-wide equalizer bands number can be changed from the equalizer settings menu

The spectrum color can be changed.

Users can now change the effects order in the left sidebar.

A new Calf Filter plugin replaces the high pass and low pass filters from previous PulseEffects versions.

Per-app level meters were removed because they caused bugs which the developer was unable to fix.

The app no longer crashes when trying to enable an optional plugin that's not installed on your system.

Fixed many potential memory leaks.

The default buffer values were increased from 100 to 200 ms, which should fix issues with noises for Bluetooth headphone users. The buffer value can be changed from the main settings menu.

Configurable number of equalizer bands

Configurable spectrum color

Download PulseEffects

PulseEffects may not work by default in Ubuntu (and other Linux distributions based on it like Linux Mint) or KDE without some tweaks. See this page for how to get it to work.

To install PulseEffects as a Flatpak package, follow the Flatpak and FlatHub quick setup guide. Then search for PulseEffects in Ubuntu/Gnome Software and install it from there, or you can install it using this command:

Update: PulseEffects now also has an Ubuntu PPA (for Ubuntu 18.04 and 18.10). The PPA has a newer PulseAudio version (because PulseEffects recommends PulseAudio >= 12) so keep this in mind before adding the PPA. Upgrading system packages may cause issues! If you can, I'd recommend using the Flatpak version of PulseEffects instead.

You can add the PPA in Ubuntu 18.10 and Ubuntu 18.04, as well as Ubuntu 18.04-based Linux distributions such as Linux Mint 19, elementary OS 5.0 Juno, and so on, and install PulseEffects, by using these commands: